A number of different types of plugs are used in well completion and stimulation operations to block fluid flow within well bores. Examples include bridge plugs and so-called “frac plugs,” which are specially designed to isolate one or more zones of a well bore during multi-zone hydraulic fracturing operations. Once fracturing is complete, plugs are removed, often by lowering a drill bit on a string of jointed pipe or coiled tubing into the well bore to mill out the plug into small pieces for circulation to the surface with drilling fluid.
A typical plug has one or more packing elements that encircle the body of the tool to form a hydraulic seal between the body of the tool and the casing or well bore. The packing elements have a diameter small enough to lower into a well bore, but are expandable, once lowered, to engage and to create a seal between the plug and a wall of a casing, liner or open wellbore. The packing elements are typically made of an elastomer that is squeezed during the setting operation to expand them, though inflatable packing elements can also be used. During a setting operation the packing elements is squeezed, causing it to expand outwardly against the wall of the well bore. To prevent the plug from moving, elements called slips, which are located along the outer diameter of tool, are pushed outwardly to engage the wall of the well casing or liner during setting operation. The slips dig into the well casing to anchor the plug within the well bore.
Plugs will often be made of cast iron and other easily drillable materials so that they may be more easily drilled than if formed of steel. So-called “composite plugs” have been developed to make it easier to mill plugs. Most of the components in a composite plug are made of composite materials or plastic rather than metal. However, composite materials are not suitable for all elements of a plug. For example, slips must typically be made of metal or ceramics, and sometimes include tungsten carbide elements to enhance performance.
Different types of rock bits have been used to mill plugs. The most commonly used drill bit type is the roller cone bit. Roller cone bits have one to four rolling cutters with cutting elements that protrude from or are disposed on the surface of the cutters. Under the weight of a drill string, the cutting elements penetrate and gouge the plug. Each rolling cutter is in the form of a cone mounted for rotation on a journal that extends from a leg that is part of the body of the drill bit at an angle that is oblique to the central axis of the drill bit. Rotating the drill bit causes the cutters to roll along a surface of the object or material being drilled. The cutting elements are usually arranged on each cone in a rows around the cone, each row being concentric with, and forming a circle or ring around, the axis of rotation of the cone. Each of the cones will typically have two or more rows of cutting elements. The outermost row of cutting elements on a roller cone cutter, nearest the outer diameter of the drill bit, is sometimes called the “heel” row. The heel row is disposed on a heel portion of the roller cone, where the conical surface of the roller cone transitions to its base. The cutting elements on the heel row cut to the gage, or outer diameter, of the bit. The base of the roller cone typically flat, but may have a beveled surface adjacent to the heel that faces the side wall of the bore, on which can be mounted inserts that act as bearing surfaces against the side wall to keep the bit straight as it is turning, as well as maintain a gauged hole. In a bit with multiple roller cones, each cone will typically have a heel row and one or more inner rows.
A number of different types of cutting elements are used on cutters of roller cone drill bits. Generally, cutting elements used to mill plugs are either (a) milled or steel teeth and (b) cemented metal carbide inserts. Generally speaking, milled teeth are better at drilling softer material and metal carbide inserts are more durable and thus better for drilling harder material.
A milled or steel tooth is made of a steel and has a generally triangular shape. Milled teeth are usually milled from the same block of steel as the roller cone. A milled tooth may be “hardfaced” with material having greater wear resistance, such as tungsten carbide particles in a metal matrix welded to the tooth, to improve wear and make the teeth more durable. A reference to milled tooth is intended to reference either a conventional or a hard faced milled tooth unless otherwise stated. Milled teeth can be made relatively long and narrow. This sharp shape allows for more aggressive gouging and scraping actions to penetrate more rapidly softer materials with low compressive strengths.
A cemented metal carbide insert is comprised of metal carbide particles cemented together with a more ductile metal binder using a sintering process to form a composite of metal carbide grains, which are typically tungsten carbide but can be other metal carbides, embedded in and metallurgically bonded to the ductile metal matrix (also called a binder phase.) Such inserts are formed into shapes and polished very smooth to reduce sliding friction. The inserts may incorporate polycrystalline diamond and similar materials, such as a wear layer, to improve wear resistance. References throughout to “cemented metal carbide inserts” include those incorporating polycrystalline diamond, cubic boron nitride and materials of similarly high abrasions resistance. Furthermore, cemented metal carbide is one type of super hard, abrasion resistant materials. References to “insert” or an insert made of “super hard material” is intended to include inserts made of cemented metal carbide, such as tungsten carbide, but also materials of similar abrasion resistance, as well as those that have substantially better abrasion resistance.
An insert is pressed into a pocket into an aperture in the roller cone body. An interference fit between the insert and the pocket may be relied upon retain the inset in the pocket. The insert may, instead, be brazed into the pocket. The insert has cylindrical portion or base, part of which is inserted into the pocket formed in the cone, and a cutting tip portion. The cutting tip portion can be formed in one of many different shapes. Conventional shapes include chisel and hemispherical or conical.
Tungsten carbide inserts are more durable than hard faced milled teeth when milling slips made of metal or other hard materials. However, as compared to milled teeth bits, bits using tungsten carbide cutters are less efficient at milling softer materials such as elastomers and composites. Further, inserts are more likely to be lost when milling plugs due to junk damage, cone shell erosion, and problems with retaining the inserts in the pockets formed in the roller cone.
U.S. patent application no. 2015/0053422 describes a rotary cone drill bit for milling plugs that purports to drill relatively harder material of a slip disposed on an outer diameter of the plug by cutter inserts disposed on an outer diameter of the bit, while the relatively softer material of the plug body is effectively drilled out by milled teeth disposed radially inward of the cutter inserts.